This invention relates to a tread pattern of the two main groove-rib type provided with auxiliary grooves having excellent stone-biting resistance and stone-expelling property.
In pneumatic tires, particularly heavy duty tires for use in heavy vehicles such as truck and bus, there are usually tread patterns of so-called two main groove-rib type wherein a pair of main grooves are formed in the tread near both side edges thereof along these edges. In addition tread patterns of four main groove-rib type exist wherein a tread width is substantially divided into five equal parts by four grooves each extending circumferentially about the tread. In this case, the two main groove-rib type tread pattern is advantageous in wear resistance and rib-tear resistance, but becomes disadvantageous in wet skid resistance, heat generation and heat dissipation as compared with the four main groove-rib type tread pattern.
In the two main groove-rib type tread pattern, therefore, there has been attempted a countermeasure such that fine grooves each having a width narrower than that of the main groove are arranged in the central region of the tread portion. In this case, however, a phenomenon inherent to the tire of the above mentioned type results, i.e. pebble stones scattered on road surface enter into the fine groove during the running of the tire. As a result, the pebble stone is frequently pushed away to the bottom of the fine groove during running of the tire, which finally results in damage of the tire casing or bursting of the tire as a worst case. But, if it is intended to expel pebble stones from the fine groove by making the opening width or angle of the fine groove large, there is a strong tendency of producing the disadvantages mentioned in the four main groove-rib type tread pattern, so that the advantages for the two main groove-rib type tread pattern cannot be given sufficiently.
In this connection, there have hitherto been proposed various countermeasures as shown in FIGS. 1-3, wherein a profile of a fine groove G in radial section of tire T is made sinuous or shelf-like protrusions are formed on the groove wall at different levels. In such countermeasures, it is hard to push away pebble stones constrained in the fine groove to the groove bottom, but it is also difficult to expel the pebble stone from the inside of the fine groove. Therefore, it has been confirmed that these countermeasures cannot basically solve the problem of producing damage of the groove bottom.